Apparatus and method for purifying alkali metal hydroxide solutions

ABSTRACT

Aqueous alkali metal hydroxide solutions are purified by passage through a porous cathode of an electrolytic cell having an anode located downstream of the cathode. An electrolytic cell of special construction particularly useful is described.

United States Patent [1 1 Otto Jan. 8, 1974 APPARATUS AND METHOD FOR PURIFYING ALKALI METAL HYDROXIDE SOLUTIONS Jack M. Otto, New Martinsville, W. Va.

Assignee: PPG Industries, Inc., Pittsburgh, Pa.

Filed: Mar. 2, 1972 Appl. No.: 231,271

Related US. Application Data Continuation of Ser, No. 806,173, March 11, 1969, abandoned,

Inventor:

[56] References Cited UNITED STATES PATENTS 2,044,888 6/1936 Overdick et a1. 204/153 3,061,537 10/1962 Yagishita 204/275 3,459,646 8/1969 Carlson 1. 204/153 3,457,152 7/1969 Maloney, Jr, et al 204/130 X Primary Examiner-John H. Mack Assistant Examiner-A. C. Prescott Attorney-Chisholm and Spencer [57] ABSTRACT Aqueous alkali metal hydroxide solutions are purified by passage through a porous cathode of an electrolytic cell having an anode located downstream of the cathode. An electrolytic cell of special construction particularly useful is described.

3 Claims, 3 Drawing Figures Nq, OH FEED LIQUOR WINTER! 8 m4 IIIIIIHHIIIIIHIII [HI SHEET 1 BF 2 PRODUCT N OH FEED LIQUOR APPARATUS AND METHOD FOR PURIFYING ALKALI METAL HYDROXIDE SOLUTIONS This is a continuation of application Ser. No. 806,173, filed Mar. 11, 1969 now abandoned.

BACKGROUND OF THE INVENTION U. S. Ser. No. 739,741 filed June 25, 1968 now US. Pat. No. 3,459,646 assigned to the assignee of this ap plication describes a process for removing metal contaminants from aqueous alkali metal hydroxide solutions in which the solutions are passed through a po rous cathode, ideally a porous carbon cathode, of an electrolytic cell while electrolyzing the solution. In the method therein specifically described, the alkali metal hydroxide solution is fed to an electrolytic cell in which the anode is disposed upstream of the cathode. Thus, prior to passing through the porous cathode the solution passes the cell anode.

The methods described in said application are highly effective in purifying alkali metal hydroxide solutions. For continuous operations, steps are provided for treating the cathode once the cathode has become so laden with contaminants (removed from the alkali metal hydroxide) such that continued passage of the solution therethrough is either difficult or nearly impossible. Special procedures restore or nearly restore the cathode permeability.

THE INVENTION This invention relates to an improvement in that method of purifying aqueous alkali metal hydroxide solutions in which the solutions are passed through a porous cathode of an electrolytic cell. Further it concerns electrolytic cell configurations particularly suited to the purification of alkali metal hydroxide solutions according to this improvement which cells are also compact, uncomplicated and economical. Thus, it has now been discovered that by disposing the anode member of the electrolytic cell downstream of the porous cathode, aqueous alkali metal hydroxide solution may be purified in an especially effective manner. Surprisingly, disposition of the anode downstream of the cathode favorably affects the porous cathode in removing metal ion contamination of the alkali metal hydroxide. Removal of metal ion contaminants from aqueous alkali metal hydroxide solutions is especially effective when the solution passes through the porous cathode prior to contacting or being near the anode.

Among the advantages is an extension of the cycle life of the porous cathode, i.e., an extension of the time period the porous cathode effectively removes metal ion contamination before requiring regeneration. A further advantage is the more effective use of the entire porous cathode.

Reference is made to the accompanying drawings and the ensuing description to facilitate the complete understanding of the present invention.

In the drawings, FIG. 1 depicts a laboratory scale electrolytic cell useful in the practice of the present invention comprised ofa cylindrical cell housing 1 within which porous disc cathode 4 and anode 3 are located. Corrosion resistant gasket 5 serves to seal cathode 4 in the cell. Standpipes 2 and 6 located on the cathode and anode side of the cell respectively provide for the removal of gaseous hydrogen and oxygen which are libertated during electrolysis. Each end of the cell housing is closed by stoppers 7 and 8. On the cathode side, the cell is provided with thermometer 9 (for measuring the temperature of feed solution) and feed liquor inlet 10. Thermo-regulator 11 for a heater (not shown) is in the catholyte compartment. Product removal line 12 is provided on the anode side. Thermometer 13 is disposed as illustrated in the anode compartment. Anode 3 and cathode 4 are electrically connected to an appropriate power source and meter (diagrammatically shown at 16) via lines 14 and 115..

The two glass half cell which comprise cell housing 1 are clamped together by use of clamping rings 18 and 21 and bolt members 23 and 24.

Feed of aqueous alkali metal hydroxide solutions through this cell is accomplished by forwarding the solution through positive displacement pump 20 into and through oil bath 119 via line into the cell. Purified solution is withdrawn from cell via line 12. In the typical operation of the cell depicted in FIG. 1, aqueous alkali metal hydroxide solution is pumped through a heating bath such as oil bath 19 into the cells catholyte compartment, then through a porous cathode into the anolyte compartment of the cell from which it is removed. Cathode 4 and anode 3 are activated by electrically connecting them to an appropriate power source while the solution is passing through the cell.

With the anode disposed downstream, standpipe 6 provides for removing anodic oxygen, thus avoiding contact of the anodic oxygen with cathode 4. Anodic oxygen deleteriously affects removal of iron contami' nation and may result in accelerating the rate at which the cathode loses permeability, thus hastening the time when the cathode requires regeneration.

The following examples illustrate practice of the present invention with the cell depicted in FIG. 1.

EXAMPLE I A cell of the type depicted in FIG. 11 having an internal diameter of 2 inches was employed, its porous cathode being provided by a graphite disc 2- /2 inches in diameter and 1 inch thick made up of number 50 pcrosity graphite.

Number 50 porous graphite has a density of 66 pounds per cubic foot, a tensile strength of 110 pounds per square inch, a compressive strength of 500 pounds per square inch and a flexural strength of 250 pounds per square inch. It has a porosity of 48 percent and an average pore diameter of 0.0019 inches. lts water permeability is 30 gallons per square foot per minute at a water temperature of 70F. and a 5 pound per square inch pressure on a one inch thick plate.

Since a portion of the porous cathode was covered by gasket 5 (as shown in the drawing) which anchored the disc into the cell, the effective diameter of the porous graphite disc was 2 inches.

A circular nickel screen just less than 2 inches in di ameter spaced approximately if; of an inch downstream of and parallel to the downstream cathode face served as the anode.

Aqueous sodium hydroxide containing 50 percent by weight sodium hydroxide was fed to this cell at the rate of 32 grams a minute after being heated in the oil bath to a temperatue of to F.

Riser 2 on the feed side of the cell was used to measure the back pressure on the porous graphite cathode. In this operation the height of the solution within the standpipe gradually increases as the cathode permeability decreases, the positive displacement pump being operated at a substantially constant rate. Regeneration of the cathode was accomplished by reversing for one hour the polarity of the electrodes, i.e., making the cathode anodic, while passing the sodium hydroxide solution through the cell and using l ampere current.

Table 1 lists the operational data and conditions.

TABLE 1 CYCLE -EE -p-pg; PRODUCT-ppm. No. Hrs. Pressure" Amps. Volts Fe Ni Cu Pb Fe Ni Cu Pb l 75 1840 mm. 0.3 2.05 3.5 0.3 0.3 1.6 0.7 0.2 0.1 0.4 2 67 1770 0.3 1.95 3.3 0.2 0.2 1.8 0.2 0.1 0.1 0.4 3 71 1780 0.3 1.95 2.6 0.5 0.3 1.5 0.2 0.3 0.1 04 4 53 1900 0.6 2.20 2.7 0.3 0.2 1.6 0.2 0.1 0.1 0.4

Anhydrous NaOH Basis In the drawings, FIG. 2 is a vertical half section of a cell whose construction is admirably suited for practice of the method of this application.

The cell comprises cell housing or cell body provided by a 3 foot high steel cylinder 3 feet in diameter, steel cell bottom and iron cell top 34. Neoprene liner completely rubber-lines cell body 30. Within cell body 30, in this particular cell, seven tubular (hollow) blind end graphite cathodes 31 are disposed in the pattern shown in FIG. 3. As illustrated in FIG. 2, the cathodes are closed at their bottom, and open at their tops. Cells are contemplated which have as few as one such cathode or have more than seven cathodes. The.

cell configuration illustrated in FIG. 2 is particularly apropos when employing two or more porous cathodes.

FIG. 3 is a cross sectional view (along 111-111) of the cell depicting the disposition within cell body 30 of the cathodes 31 and anodes 33.

Cathodes 31 are threaded into liquid and gas impervious graphite (Karbate) tube sheet 32. Rubber linings 41 and 50, respectively, insulate electrically cell cover 34 and bottom 35 from cell body 30 and cathodes 31.

Each cathode, in the particular cell, is a 36 inch long cylinder 6-% inches in outer diameter and has an inner diameter of 3 inches. Thus, the cathode walls and bottoms are l-7/l6 inches thick.

Disposed concentrically in the hollow center of each cathode 31 is an anode 33. These anodes are made of a hollow nickel tube with an outer diameter of 1.315

inches, the lower ends of which are welded shut. Their upper ends are anchored by welding (but may be threaded) into cover 34. In operative positions, the anodes reach to within about 1.2 inches of the inner surface of the cathode bottom: i.e., anode surfaces are spaced equidistantly from the inner surface of the cathode in which the anode is disposed.

Cathode electrical connection involve two copper bars 46 connected to tubesheet 32 by bolt pairs 36 and 38; and 37 and 39. Cathode cables 44 are connected to a rectifier (not shown). In this fashion, the current is distributed over tubesheet 32 from four equispaced points.

Anode cables 51 are connected electrically to cell top 34 using bolt pairs 52 and 53.

The cell shown in FIG. 2 is especially convenient to assemble and disassemble, as the case may be. In astop 34 and spacing ring 56 the undersides of which are rubber-lined and into which top the depending anodes 33 are screwed or otherwise attached) is appropriately aligned and lowered into the combination of the cathode assembly and cell housing so that the anodes fit into the hollow interiors of the cathodes. The rubberlined outer underside edge of cover 34 seats suitably atop the upper surface of the cell housing top. As shown in FIG. 2, rubber lining 50 extends as a continuous insulatory lining over the underside periphery of cover 34, and around and under spacer ring 56. As shown, spacer ring 56 circles bottom outer periphery of cover 34. In this fashion, the bottom of the cover (and lining) is spaced upward from tubesheet 32. This provides for space 55 into which liquor which has passed through the cathodes and up through the hollow centers can be collected for withdrawal via exit pipe 43. Other spacing means obviously maybe used. Similarly, insulating (electrical) materials which are resistant to the conditions prevailing in the cell other than rubber can be used.

Use of this cell to treat alkali metal hydroxide solution pursuant to this invention involves feeding the solution to the cell through inlet 42 (while the electrodes are electrically connected). This fills the cell with solution which passes through the cathodes into the center thereof and up the annuli provided between the anode surface and inner surface of the cathode. As the solution passes through the cathode, metal ion contaminants are removed (by deposition in the cathode). So purified solution then travels up the annuli to the top of the cell wherefrom it is removed via outlet 43. Drain 45 provies a convenient expedient for periodically removing liquor, for example, as part of the regeneration procedure, or it can be used in lieu of inlet 42 to feed k m a hyd xidssslutisan-..

Thus, in operation of the cell depicted in FIG. 2, the aqueous alkali metal hydroxide is in effect divided into a plurality of streams (seven in this specific cell) each of which is simultaneously treated by passage through a porous cathode. That is, this cell configuration treats solutions within a single cell tube (with simple electrical connections) as if seven single cells were used in parallel. Electrically each cathode-anode pair in the cell is in parallel to each other such pair. This provides for highly efficient operation.

I In utilizing such cell, aqueous sodium hydroxide solutions (50 weight percent NaOH) are obtained containing less than 1.5 (often about 0.8 to 1) parts per million iron (feed of 4 or 5 ppm) no more than 0.! part per million nickel (feeds of 0.8 ppm) and less than 0.4 part per million lead (feeds of 2 ppm) on a NaOH (anhydrous) basis with feed rates of about gallons per minute. Cycle times on the order of 100 hours are feasible, for example, using No. 45 porous graphite for the cathodes. This represents a significant improvement in cycle life over a comparable cell configuration, but wherein the anodes are upstream (contrasted with downstream) from the cathodes.

Using No. 25 porous graphite (more porous than No. 45), cycle times of 200 hours are attainable, again longer than cycle times with the anode located upstream.

Other useful metals for cathodes include zirconium, molybdenum, silver and Hastalloys. Less desirable, but

operative metals include steel, cast iron, chrome plated steel and tungsten. These less desirable metals are pri' marily appropriate under only cathodic conditions and with alkali or non-acidic regenerations.

Of prime consideration in selecting materials for the cathode is that the cathode possesses a certain and definite permeability. Preferably, for example, porous Physcial Properties of Porous Graphite Average Density Strength lbs/ft Pore Diameter 7 Permeability" Grade No. lbs/ft Tensile Comp. Flexural Inches Gal/ft'lmin.

25 64 70 400 200 .0047 90 45 65 I50 500 300 .0023 60 66 200 600 400 .OOl3 l0 All have a poronity of 48 percent. Water at 7(J"F.---5 pru' pressure -plule 1 inch lhick.

This improvement (extension in time the cathode can be used before requiring regeneration) in the cathodes capacity to remove metal impurities brought about by disposing the anode downstream of the cathode is at least in part attributable to the increase in the distribution of impurities throughout the cathode thickness.

Comparable results (improvements) obtain with feed rates of 7. to 30 gallons per minute, caustic solution temperatures of l50190F. and cell currents of 600 to 1300 amperes.

Once permeability of the cathodes reach predeter mined level (one at which further treatment is deemed uneconomical), regeneration to restore an effective level of cathode permeability is necessary. One particu lar expedient involves shutting the cell down, draining its contents, water washing the cell, then reversing the cell polarity (i.e., operating the graphite cathode as an anode) while it is filled with hydrochloric acid (e.g., 3 to 25 weight percent l-lCl). Typically, the acid is allowed to sit in the cell for l to 2 hours. Then the acid is drained, the cell water-washed and restored to its normal operating conditions, to wit, cell polarity reestablished so that the graphite electrodes are cathodic. It is possible also to flow appropriate regenerating liquid through the cell in the path the hydroxide follows or by reversing the flow, i.e., feeding through pipe 43 and removing via pipe 42.

As disclosed in commonly assigned application Ser. No. 739,741 (the disclosure of which is hereby incorporated by reference), the treatment may be applied to aqueous alkali metal hydroxide solutions of any strength. While the treatment is particularly useful with solutions containing to 73 weight percent sodium hydroxide, more dilute solutions also can be treated with benefit. Besides sodium hydroxide solution, potassium and lithium hydroxides, too may have their metal ion contamination reduced.

graphite cathodes should possess a minimum permeability to water at F. of at least 14 gallons per square foot per minute with 5 pounds per square inch gauge pressure applied to a plate of the cathode material one inch thick, preferably a permeability considerably greater than this, for example, 30 to gallons per square foot per minute. With a porous cathode material of lesser permeability, for example, one with permeability of say 10 gallons per square foot per minute, a reduction in cathode thickness can be used as a partial compensation. Porous electrode material which possesses a tensile strength of 60 pound per square inch or greater is typical of material which may be employed.

Also of importance in the efficiency with which the cell removes metal ion impurities, is the intimacy of contact between the alkali metal hydroxide solution and the cathode. Thus, it is not only important the solution pass through the cathode, but it is preferable there be more than momentary and casual contact. To this end, typical carbon thicknesses range upwardly of about 0.2 inches to 5 or more inches with average pore diameters in the cathode ranging downwardly of 0.01 inches, for example, to as low as about 0.001 inches. Residence times within the porous cathode will be in excess of 5 or 10 seconds, often even as long as 5 or 10 minutes. With some cathode materials structurally stronger than carbon, cathodes somewhat thinner than 0.2 inch are possible.

In operating these cells, cell voltage and amperes necessary to plate out the quantities of metal ion impurities contained in the caustic liquor being treated are employed. Generally speaking the cells will range in voltage from l-% to 5 volts. The particular voltage used satisfactory deposition of the metal ion impurities contained in an alkali metal hydroxide solution will be obtained. Current densities can range upwardly from about 10 or 13 to 400 amps per square foot or more. At the higher current densities gas binding is apt to occur at the cathode surface; increasing the flow rates of the liquor being purified when these higher current densities are employed assists in avoiding the consequences of such gas binding. Current densities below 10 amps per square foot, e.g., in the range of l to 5 amps per square foot may be employed. Usually it is then advisable to provide a downward adjustment in liquid flow rate through the cell.

As previously indicated, the porous cathode experiences a continuing loss of permeability during cell operation until it reaches that level at which further operation is no longer deemed justifiable. When this condition is reached (e.g., the pressure drops across the cathode is no longer satisfactory) it is necessary to regenerate. This preferably involves reversing the cell polarity so that the porous cathode becomes an anode and the cell anode becomes a cathode. While this reversed polarity condition is in operation the caustic liquor flow may be maintained as it was before polarity reversal or its direction of flow may be reversed so that it flows in a direction opposite to its flow before reversal. Since the cell is operating electrolytically in reverse, metal ion impurities contained in the pores of the cathode as metal find their way into the caustic solution as ions once again and become redissolved. Some impurities, such as nickel, in the pores at least in part are removed as collodial or peptized solid using alkali metal hydrox-' ides for regenerations. This reversing of the cell current is conducted for periods of time ranging from 15 minutes to 2 hours or longer if necessary until the graphite or carbon electrode material returns to or close to its original permeability. Upon attaining a satisfactory level of permeability, typically when the permeability is at or close (within 10 to 30 percent) to the original permeability of the electrode, the current and flow reversals are discontinued; the cell is then ready to return to operational conditions used to remove ion contamination.

Any electrolyte which will not unduly corrode the cell equipment and in which the metal ions removed on the cathode of the cell are soluble can be utilized during current reversal. Thus, for example, electrolytes other than'hydroxides of alkali metals can be used. For example, aqueous solutions of sodium chloride, potassium chloride, lithium chloride, ammonium hydroxide, aqueous mineral acid solution such as hydrochloric acid, sulfuric acid, nitric acid, phosphoric acid and chromic acid may be employed. Indeed any electrolyte in which the metals normally contained on the cathode of the cell would be considered soluble and which at the same time would not be damaging to the electrode materials can be used.

Current reversal removal of metal from the pores of the cathode is typically but not necessarily (and conveniently) conducted at the same current density that the cell was operated when the metals were plated out. Current density can be lower or higher, however, than the current density employed during purification. At lower current densities, regeneration takes longer while at higher values the rate is considerably faster. As with plating out the metals, high current density can give rise to gas binding; increasing the liquor flow rate will help minimize any such effect if it occurs.

Current reversal is not always essential to regeneration. Stopping current flow or reducing the current flow so that the potential on the cathode is below that at which the metals deposit out can be used. Iron, for example, can be removed from a porous carbon cathode (to effect regeneration) while the current is off or the potential is reduced to an appropriate level. However, current reversal speeds up the rate of iron removal.

Alkali metal hydroxide solutions treated hereby typically are contaminated with metal ions on a tens of parts per million level or less (on an anhydrous alkali metal hydroxide basis).

Generally speaking, iron levels on the order of approximately 3 to 4 parts per million are typical of those encountered and these may be effectively treated to remove the iron ion contamination to a level of 1 part per million or lower. However, the process will effectively treat solutions with iron levels much higher, e.g., 12 to 15 parts per million by weight of NaOH on an anhydrous basis. In addition to iron metal ions such as nickel, copper, manganese, calcium, magnesium, aluminum, chromium, lead, vanadium, molybdenum, titanium, beryllium, zirconium, tungsten, tin, mercury, strontium and barium are apt to be in the alkali metal hydroxide solutions. The concentration of many such metal ions found in alkali metal hydroxide solutions may be reduced materially. For example, in a caustic soda solution containing as little as 03 part per million nickel metal ion contamination may be reduced to a level of 0. 1 part per million. With caustic soda solutions having as little as 0.2 part per million copper therein, copper may be reduced to 0.1 part per million. Lead ion contamination at the levels of 2.7 parts per million may be reduced to 0.4 part per million or lower.

While the invention has been described with reference to certain specific examples and illustrative embodiments it is of course to be understood that it is not intended to be limited thereby except insofar as appears in the accompanying claims.

I claim:

1. In a method of reducing metal ion content of aqueous alkali metal hydroxide solutions by subjecting metal ion contaminated alkali metal hydroxide solutions to electrolysis in an electrolytic cell having an anode and cathode pair while passing the solution through and in intimate contact with a liquid permeable cathode of the cell to deposit out metal ion contamination in said cathode, the improvement which comprises passing substantially oxygen free alkali-metal hydroxide solution first through a porous cathode, depositing metal within the cathode in the substantial absence of oxygen, then passing the solution out of the cathode towards the anode of said electrode pair, and thereafter and without being subjected to further electrolysis in the presence of any electrode withdrawing said solution from said cell.

2. The method of claim 1 wherein the alkali metal hydroxide is sodium hydroxide.

3. A method of removing metal ion impurities from aqueous alkali metal hydroxide solution which comprises passing metal ion contaminated, substantially oxygen free, alkali metal hydroxide solution through the pores of and in intimate contact with porous walls of a hollow cylindrical porous cathode having an anode located within its walls while subjecting the solution to electrolysis in the substantial absence of oxygen to de from the cell by directly withiirawing it from within the posit out the metal ion contamination in the pores of hollow cathode without further contact with any electhe cathode wall, and thereafter removing the solution tr des within the cell. 

2. The method of claim 1 wherein the alkali metal hydroxide is sodium hydroxide.
 3. A method of removing metal ion impurities from aqueous alkali metal hydroxide solution which comprises passing metal ion contaminated, substantially oxygen free, alkali metal hydroxide solution through the pores of and in intimate contact with porous walls of a hollow cylindrical porous cathode having an anode located within its walls while subjecting the solution to electrolysis in the substantial absence of oxygen to deposit out the metal ion contamination in the pores of the cathode wall, and thereafter removing the solution from the cell by direcTly withdrawing it from within the hollow cathode without further contact with any electrodes within the cell. 